1. Field of the Invention
This invention relates to magnet-superconductor systems in which the levitation and suspension of either the magnet or the superconductor can be controlled, and more particularly to such a system utilizing a current-carrying coil as a control element to provide selective and controllable levitation and suspension effects.
2. Description of the Related Art
With the discovery by Bednorz and Mueller of superconducting effects at higher temperatures, many experiments have been performed relative to the levitation of a magnet above a superconductor cooled to liquid nitrogen temperatures. In these experiments, a superconductor can freely float above a permanent magnet, or vice versa, as a demonstration of superconductivity at elevated temperatures. This effect is seen in type I superconductors and also in type II superconductors having no flux penetration below H.sub.c1. The Meissner effect is used to explain this levitation.
The following technical articles describe levitation in magnet-superconductor systems and explain the effects occurring with both type I and type II superconductors.
1. E. H. Brandt, Appl. Phys. Lett. 53 (16), p. 1554, Oct. 17, 1988. PA1 2. F. Hellman et al, J. Appl. Phys. 63 (2), p. 447, Jan. 15, 1988.
While it had been generally considered necessary to use a type I superconductor or a type II superconductor below H.sub.cl. where a complete Meissner effect exists, levitation can work equally well with a type II superconductor between H.sub.c1 and H.sub.c2. In both systems, the height of the levitation depends on variables such as the thickness of the superconducting disk and the size of the magnet. Hellman et al used a model based on the energy costs of flux penetration through vortices in the superconductor to explain levitation with type II superconductors between H.sub.c1 and H.sub.c2.
The Brandt technical paper describes the presence of friction in systems using type II superconductors, the friction enabling a continuous range of stable equilibrium positions and orientations to be obtained in the floating magnet or superconductor. This strong internal friction indicates the existence and unpinning of flux lines in the superconductors. The magnet force on levitated type II superconductors even with weak pinning of flux lines is shown to be hysteretic, providing a stable, almost rigid levitation and a continuous range of stable positions and orientations which increases with increasing pinning strength.
In addition to these interesting levitation effects, suspension effects have been observed in systems utilizing high T.sub.c superconductors and permanent magnets. An early paper describing the stable suspension of a silver oxide doped high T.sub.c superconductor below a permanent magnet is P. N. Peters et al, Appl. Phys. Lett. 52 (24), p. 2066, Jun. 13, 1988. The oxide superconductor used in the experiments of Peters et al was YBa.sub.2 Cu.sub.3 O.sub.x into which silver oxide was added to limit intergrain resistances which limit high critical currents. Peters et al provide a model for the suspension effect, explaining it in terms of many current loops with weak links surrounding vortices. When the silver oxide doped 1-2-3 superconductor is moved in the field of a permanent magnet the induced currents may exceed the critical currents in some of the loops and allow flux to penetrate or to leave the superconductor. The flux trapped in the superconductor produces a force which tends to hold the sample at a fixed position in the external magnet field.
W. G. Harter et al, Appl. Phys. Lett. 53 (12), p. 1119, Sep. 19, 1988, describe levitation and suspension effects using high T.sub.c thallium-based superconductors. These superconductors provide an effect similar to those found in Y-Ba-Cu-O superconductors, and exhibit very stable suspension or levitation equilibrium positions.
Another paper describing the magnetic hysteresis effect in silver oxide doped superconductor-permanent magnet systems is C. Y. Huang et al, Mod. Phys. Lett. B, 2, 869 (1988). This is a follow-up paper to the P. N. Peters et al paper first reporting the suspension effect using silver oxide doped YBa.sub.2 CuO.sub.x superconductors. Magnetic suspension effects in superconductors at 4.2K are described by R. J. Adler et al in Appl. Phys. Lett. 53 (23), p. 2346, Dec. 5, 1988. The superconductors used in this experiment were NB.sub.3 Sn and undopod YBa.sub.2 Cu.sub.3 O.sub.x.
In a paper entitled "Flux Penetration in High T.sub.c Superconductors: Implications for Magnetic Suspension and Shielding" published in Appl. Phys. A48 , pp. 87-91 (1989) by D. D. Marshall et al, both levitation and suspension effects are described. Focusing of the magnetic field lines by the superconductor is noted, as well as the existence of stable equilibrium suspension. These stable equilibria are shown to be related directly to hysteresis observed in the force-separation relation for a magnet and the superconductor. Observations were made that the levitation height of a magnet increases with magnet size, which is contrary to what would be expected from the literature.
While the references describe both levitation and suspension of a magnet with respect to a superconductor, and the attainment of a range of positions, no teaching or suggestion is made for the attainment of a continuous range of positions using an external control. In a practical system, it would not be desirable to have to reach into the system to manually change the position of the magnet relative to the superconductor. Rather, what is needed is a control for providing selectivity so that a continuous range of positions can be achieved, the range being greater than the limited range available using the techniques described by these references. Thus, it is a primary object of the present invention to provide an apparatus for modifying the attractive and repulsive interactions in the superconductor-magnet system to allow more stable and controllable levitation and suspension effects.
It is another object of the present invention to provide a magnet-superconductor system in which an external control is used to provide a continuous range of stable relative positions in the magnet-superconductor system, and also to provide stable rotational effects about an equilibrium position.
It is another object of this invention to provide a structure and technique for increasing the pinning forces in the superconductor of a superconductor-magnet system, by modifying the magnetic field of the magnet-superconductor system.
It is another object of this invention to provide a magnet-superconductor system in which the attractive and repulsive forces existing between the magnet and the superconductor can be varied over a continuous range, the range being greater than that which would exist without the external control.
It is another object of this invention to provide a three-way system including a magnet, a type II superconductor, and an external control in which the orientation of the magnetic moment of the levitated or suspended magnet is adjustable over a range of angles.
It is another object of this invention to provide a magnet-superconductor system including magnetic field control means which makes it possible to suspend a much higher mass than has been previously possible.